Conventional bicycles are traditionally driven by a chain. The chain transmits mechanical power from a crank sprocket to a rear sprocket. However, there are several disadvantages of using chains. First, chains require lubrication. Second, chains wear and become less efficient in transmitting power over time. Third, chains stretch with extended use and thus need to be replaced periodically. Fourth, chains may wear the teeth of the crank sprocket or rear sprocket requiring their replacement. Thus, the ability to use flexible belts on bicycles is desirable because belts offer increased wear resistance, quieter operation, and lubrication-free cleanliness. However, there are several issues that arise with the use of flexible belts.
First, unlike chains, belts are manufactured in a continuous loop. This creates an installation problem because belts cannot be broken to pass through a bicycle frame. Therefore, it is necessary to create a split in the frame through which the belt can pass. The split must be closeable, durable, and designed in such a way to maintain the structural integrity of the frame.
Second, similar to chains, belts must be tensioned to provide efficient transfer of mechanical power from the crank sprocket to the rear sprocket. Currently, several chain tensioning methods are available; however, all of the available options have drawbacks when applied to belts.
For conventional bicycles with multiple sprockets at the hub, a spring-loaded derailleur may be used to maintain the tension of the chain and to shift the chain between sprockets. However, on belt-driven bicycles, the belt alignment between the crank sprocket and the rear sprocket is critical. Thus, spring-loaded derailleurs are not desirable because they increase the risk of misalignment. Additionally, spring-loaded derailleurs are not preferred by some riders because of their bulkiness.
Horizontal dropouts are another chain tensioning alternative. Horizontal dropouts, more commonly known as track ends, are the oldest method of tensioning a chain for a single speed drive train on a bicycle. The rear axle is mounted in a horizontally elongated dropout. Sliding the rear axle back and forth within the horizontal dropout changes the distance between the rear sprocket and the crank sprocket, thus changing the tension of the chain. However, wheel misalignment is somewhat common in horizontal dropouts because of axle slippage. Additionally, it can be difficult to align disk brakes on bicycles that have horizontal dropouts because adjusting the rear axle to tension the chain causes the wheel-mounted disk brake to de-align and ultimately lose contact with the frame-mounted brake caliper.
Eccentric bottom brackets provide yet another chain tensioning alternative. An eccentric bottom bracket offsets the axis of the crank bearing from the center axis of the bottom bracket and the center axis of the bottom bracket shell. Rotating an eccentric bottom bracket about its center axis changes the distance between the crank sprocket and the rear sprocket, thereby changing the tension of the chain. However, eccentric bottom brackets are notorious for being creaky, which can be rectified, but usually requires a tear down of the bottom bracket assembly. Additionally, eccentric bottom brackets are heavier than other chain tensioning alternatives.
What is needed is a bicycle dropout that accommodates installation of a belt or chain though a bicycle frame, provides an efficient means of tensioning the belt or chain to maximize power transmission from the crank sprocket to the rear sprocket, removes the need of realigning disk brakes every time the belt or chain is tensioned, and reduces slippage of the axle after the belt or chain is tensioned.
The various embodiments described herein are intended to overcome one or more of the problems discussed above.